Notify users when an extension update is available

[Jorriss]

Summary

SSMS 22 has no extension-management UI, so users have no built-in way to learn that a newer Statistics Parser has been released. This adds update-availability notification (no auto-updating — notification only).

Full design: docs/UPDATES.md.

Scope

• Version source ("Option B") — a latest-version.json committed to the repo root, served from the GitHub raw CDN (avoids the rate-limited GitHub API).
• Passive notification — a dismissible InfoBar-style banner at the top of the Parse Statistics tab when a newer version is known. Network fetch throttled to once per 24h; throttle/dismiss state persisted to %LocalAppData%\StatisticsParser\update-state.json.
• Active notification — the About window auto-checks on open and offers a manual Check for Updates button, alongside the current version number.
• Opt-out — a "Check for Updates" toggle on the Tools > Options page that gates automatic checks; the manual About button still works when it is off.

Implementation notes

• JSON parsed with DataContractJsonSerializer (no new package dependency), matching the Core project's no-external-deps style.
• New source/StatisticsParser.Vsix/Updates/ namespace: UpdateInfo, UpdateCheckResult, VersionManifest, UpdateState, UpdateStateStore, UpdateCheckService.
• Version comparison via System.Version (never string compare); current version from AssemblyFileVersion.
• Update checks are strictly best-effort — every failure path is swallowed and never delays SSMS startup.

Acceptance criteria

• Up-to-date: About shows "You have the latest version."; no banner.
• Update available: banner appears on the Parse Statistics tab; Dismiss hides it until the next release; About shows "Version X is available" with a working Download link.
• Opt-out: unchecking "Check for Updates" in Options suppresses the banner and the About auto-check; the manual button still works.
• Failure path: offline/proxy failures surface as "Couldn't check for updates." in About and never disrupt SSMS.